The present invention relates to a process for crystallizing the adduct of bisphenol A with phenol.
There is an increasing demand for bisphenol A as a raw material for polycarbonate resins and epoxy resins, particularly for engineering plastics. These applications need colorless and high-purity bisphenol A.
Bisphenol A is produced, for example, by reacting phenol with acetone in the presence of an acid catalyst, freeing the product mixture of the catalyst, water, unreacted acetone, and a small amount of phenol, cooling the remaining liquid mixture, thereby crystallizing the adduct of bisphenol A with phenol, separating the adduct crystals from the mother liquor, and removing phenol from the adduct, thereby obtaining bisphenol A.
In the case where the catalyst is hydrochloric acid, the product mixture is heated to 100 to 120.degree. C. under reduced pressure for the removal of hydrochloric acid, unreacted acetone, water, and a small amount of phenol.
The vacuum distillation is usually accomplished by controlling the temperature of the bottom product and thus controlling the concentration of the bottom product, while keeping the operating pressure constant according to the vapor-liquid equilibrium of phenol and bisphenol A. (In this case, the bottom product is regarded as a binary system composed of phenol and bisphenol A.)
A disadvantage of this operating method is that the concentration control by means of the temperature control is practically difficult because the boiling point of the bottom product changes only a little even when the concentration of bisphenol A changes greatly. For example, the boiling point is 107.degree. C., 108.degree. C., 109.degree. C., and 110.degree. C. when the concentration of bisphenol A is 25 wt. %, 30 wt. %, 35 wt. %, and 40 wt. %, respectively, if the distillation pressure is 50 mm Hg. Therefore, it is difficult to keep constant the concentration of bisphenol A in the bottom product.
If the bottom product with fluctuating concentrations is continuously fed to a crystallizer, the amount of crystals that are produced in the crystallizer will fluctuate. This makes the quality of the adduct of bisphenol inconsistnet thereby A with phenol, adversely affecting the quality of bisphenol A.
In addition, the fluctuation of concentration leads to the great fluctuation of particle size. This, in turn, leads to the fluctuation of quality because the crystals carry the mother liquor containing impurities in the solid-liquid separating step, with the amount of the mother liquor carried varying depending on the particle size.
The fluctuating concentration of bisphenol A in the slurry poses another problem. An excessively low concentration leads to low yields. An excessively high concentration leads to an increased slurry viscosity, making the slurry transportation impossible.
A problem associated with the continuous crystallization is the deposit of scale on the inside wall of the crystallizer. The deposit of scale interrupts the operation of the crystallizer, making it impossible to produce crystals of uniform quality in a stable manner.
The crystallization of the adduct of bisphenol A with phenol may be accomplished by a process disclosed in Japanese Patent Laid-open No. 135832/1983. According to this process, heat including the heat of crystallization is removed by adding water and evaporating the water. It is considered that no scale easily deposits on the inside wall of the crystallizer because the heat of crystallization is removed internally. Incidentally, said Japanese patent describes nothing about lagging the crystallizer.
The deposit of scale in the crystallizer is usually prevented by providing the crystallizer with a lagging material or a jacket for hot water circulation. These provisions prevent the degree of supersaturation from excessively increasing on the inside wall. The deposit of scale is also prevented by providing the crystallizer with a scraper which removes the scale from the inside wall, or by adding a solvent which dissolves the scale. (See Chemical Engineering Handbook, 4th edition p. 453, published by the Japanese Chemical Industry Association.)
In the case where the adduct of bisphenol A with phenol is crystallized by adding water, providing the crystallizer with a jacket has a disadvantage that a large amount of vapor is generated when the jacket is kept at an excessively high temperature. This leads to an energy loss and makes it necessary to enlarge the equipment. In addition, this causes the vigorous vaporization and bumping of water, which disturb the crystal growth, resulting in the decreased purity and particle size of the crystallized adduct.
On the other hand, providing the crystallizer with a scraper has a disadvantage that the scraper crushes the crystals, making it difficult to separate the crystals in the subsequent solid-liquid separating step, and resulting in decreased quality (due to the mother liquor remaining on the crystal surface) and decreased yields.
Adding a solvent to prevent the deposit of scale is not economical because an additional apparatus is necessary for solvent recovery. In addition, the solvent added is liable to deteriorate the quality of the product.